Google has pushed new code to AOSP (Android Open Source Project), marking it with 2 new tags: android-4.2.1_r1.1 and android-4.2.1_r1.2. The build number corresponding to the 4.2.1_r1.2 release is JOP40G, with mentions of JOP40F and JOP40E along the way, too.
There are no major changes included, and Bluetooth fixes will have to wait for the next release (4.2.2). All changes seems to relate to the Nexus 4, fixing some of the known bugs while tweaking things along the way.
It’s a developer’s thing and most of us won’t be able to use any code from here. If you’re interested to dig deeper though, feel free to hop over to these two links and get the code you need… Alternatively (or in addition), you can first read the change log over at AndroidPolice. It’s your call.